Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $34.88   8.212%   $37.7443   $37.74   $37.75   $37.70  
  Purchase #2 $27.41   7.205%   $29.3849   $29.38   $29.40   $29.40  
  Purchase #3 $22.78   5.469%   $24.0258   $24.03   $24.05   $24.00  
  Purchase #4 $161.96   11.738%   $180.9709   $180.97   $180.95   $181.00  
  Purchase #5 $129.83   10.144%   $143   $143.00   $143.00   $143.00  
  Purchase #6 $211.53   12.467%   $237.9014   $237.90   $237.90   $237.90  
  Purchase #7 $109.81   7.492%   $118.037   $118.04   $118.05   $118.00  
  7 purchase totals = $698.20       $771.0643   $771.06   $771.10   $771.00